N/N/O versus N/O/O and N/O amino isoborneols in the enantioselective ethylation of benzaldehyde

Enantiopure 10-(4-methylpiperazin-1-yl)isoborneol, a N/N/O-tridentate chiral diamino isoborneol, has been obtained from enantiopure ketopinic acid and its catalytic activity (as chiral ligand in the enantioselective addition of diethylzinc to benzaldehyde) measured and compared with the previously reported data for related N/N/O, N/O/O, and N/O isoborneols. It is demonstrated that the pentacoordinated-zinc Oppolzer's model for transition states is useful for explaining the catalytic behavior of N/N/O-tridentate ligands. In such cases, stable syn- or anti-type transition states can be proposed depending on the catalyst's conformational flexibility.